My passion for lifelong learning. The only way to truly understand a subject is the capability to explain it to other people. When I was a high school student, I volunteered as a tutor for fellow students having trouble with math and physics. I often got the feeling that not only did they profit from it, but I did so myself. Significantly. I got a much deeper understanding of the more difficult stuff, because I was forced to explain it in different ways.
Second reason: I love English.
Third reason: Being part of a (global) community.